1. Field of the Invention
The present invention relates to a multi quantum well semiconductor laser. In particular, the present invention relates to a multi quantum well semiconductor laser suitable for a light source for optical communications used in a CATV and the like, in which signals are transmitted by a plurality of carrier waves, and to an optical communication system using the multi quantum well semiconductor laser as a light source for information transmission.
2. Description of the Related Art
In recent years, an analog multiple light transmission system has been introduced into the field of image transmission, particularly, a CATV. In such an analog multiple light transmission system, the transmission of light (40 channels, about 10 km) using AM modulation has reached a practical level. As a light source used in the system, a long wavelength bandwidth semiconductor laser having a low noise, low transmission distortion characteristic is required. An example of such a light source includes a distributed-feedback semiconductor laser (hereinafter, referred to as a DFB laser) using an InGaAsP/InP material.
In the case where the number of channels increases to 40 or more, the modulation frequency bandwidth is required to be 300 MHz or more. It is known that the increase in modulation frequency bandwidth rapidly increases frequency distortion because modulation frequencies close to a relaxation oscillation frequency; as a result, a low transmission distortion characteristic cannot be obtained (Electronics Letters Vol. 28 No. 9 (1992) 891).
Under the above-mentioned circumstances, in order to widen a modulation frequency bandwidth and achieve a higher relaxation oscillation frequency, a DFB laser having a multi quantum well structure as an active layer has been fabricated.
FIG. 6 shows a structure of a representative 1.3 .mu.m wavelength DFB semiconductor laser used for 40 channel-10 km transmission. The DFB laser includes an n-type InP substrate 1 and a stripe-shaped cavity formed on the substrate 1. The stripe-shaped cavity includes a diffraction grating 2 with a period of 200 nm formed on the n-type InP substrate 1, an n-type InGaAsP optical wave guide layer 12 (thickness: 0.15 .mu.m, bandgap wavelength (composition wavelength) .lambda. g=1.15 .mu.m) , a 10-layered quantum well active layer 4, a p-type InGaAsP optical wave guide layer 13 (thickness: 30 nm, .lambda. g=1.15 .mu.m), and a p-type InP cladding layer 6. The quantum well active layer 4 includes InGaAsP well layers 14 (each layer thickness: 4 nm, bandgap wavelength .lambda. g=1.4 .mu.m) and InGaAsP barrier layers 15 (each layer thickness: 10 nm, bandgap wavelength .lambda. g=1.15 .mu.m ).
The reason for using InGaAsP having a bandgap wavelength of .lambda. g=1.15 .mu.m as material for the optical wave guide layer 12 and the barrier layers 15 is as follows. Light is sufficiently confined in the quantum well active layer 4 because of its material's relatively high refractive index. As a result, a low threshold current characteristic can be obtained. The stripe-shaped cavity is buried from both sides with current blocking layers made of p-type InP 16, n-type InP 17, and p-type InP 6. The length of the stripe-shaped cavity (cavity length) is 300 .mu.m, and its width is 1.2 .mu.m. Each end facet of the cavity is covered with an appropriate dielectric film, whereby the reflectance on the light-emitting face and that on the face opposite to the light-emitting face are set to be 5% and 70%, respectively.
FIG. 9 is a graph showing the relation between a composite second order distortion (CSO) and a bias current dependence of an optical fiber output (Pf) from a module. Herein, the CSO and the bias current dependence are obtained when the above-mentioned semiconductor laser is provided in the module having an isolator, and 58 channels of TV signals are transmitted for 10 km through optical fibers in an NTSC system. The CSO was detected at 55 MHz where the number of composite distortions becomes maximum.
In this graph, a curve (a) represents a characteristic of a satisfactory semiconductor laser. In this semiconductor laser, the optical fiber output can be obtained in such a wide range as 2 to 6 mW, and a low transmission distribution characteristic (i.e., CSO&lt;-60 dBc) can be obtained. In addition, 53 dB or more of noise (CNR) characteristic and -65 dBc or less of a composite third order distortion (CTB) can be obtained. Accordingly, this semiconductor laser satisfies the qualities required for a high image quality CATV.
However, the above-mentioned conventional semiconductor laser has the following problems.
Among semiconductor lasers lasing in a single mode, the number of semiconductor lasers having a characteristic as represented by the curve (a) of FIG. 9 is 10% or less. Most of the semiconductor lasers actually have characteristics as represented by curves (b) and (c) of FIG. 9. This means that each semiconductor laser is different in CSO.
This is due to an axis-direction hole burning effect. The axis-direction hole burning effect is caused by the fluctuation of a refractive index of a laser cavity during modulation. The degree of the axis-direction hole burning effect mainly depends upon the height and phase of the diffraction grating at the end facets thereof, the reflectance on the end facets of the laser cavity, and the like. The height and phase of the diffraction grating, and the reflectance on the end facets of the laser cavity are varied depending upon the kind of semiconductor laser; thus, the characteristics of the semiconductor lasers are also varied.
In an actual fiber transmission path, a plurality of light connectors are present, causing the reflection on the end facets of the light connectors. Between such a plurality of reflective points, multiple reflection is caused. It is pointed out that when the wavelength fluctuation is large during modulation of the DFB laser which is a light source, additional noise and transmission distortion are caused because of the effects of multiple reflection (J. Lightwave Technol. 9 (1991) 991). Hereinafter, such a wavelength fluctuation is referred to as "chirping".
The amount of chirping of the conventional DFB laser is large, i.e., 200 MHz/mA or more at a modulation frequency of 1GHz or less. Because of this, additional noise and transmission distortion are caused.
The amount of chirping (.DELTA. F/.DELTA. I) of a semiconductor laser is represented by the following expression: EQU .DELTA. F/.DELTA. I=F0..GAMMA./(L.Nw.Lz.W) (1)
where .GAMMA. is an optical confinement factor in a well layer, L is a cavity length, Nw is the number of well layers, Lz is a well width, W is a stripe width of an active layer, and F0 is a term of a proportional coefficient. Assuming that L, W, and Lz are constant, in order to decrease the amount of chirping (.DELTA. F/.DELTA. I), the term .GAMMA./Nw should be made small.
By decreasing Nw from 10 (conventional example) to 5, .GAMMA./Nw is decreased to 80%. Thus, the amount of chirping (.DELTA. F/.DELTA. I) is expected to be decreased by a similar degree.
FIG. 7 shows a chirping characteristic (modulation frequency fm=200 MHz) of DFB lasers having the same structure as that of the conventional example, with varying of the number of wells (5, 7, and 10). Herein, the bias current is set to be Ith (threshold current)+30 mA. As is understood from FIG. 7, the amount of chirping increases with the decrease in the number of well layers; thus, the characteristic expected from Expression 1 cannot be obtained.
In the field of future image transmission, the introduction of FM modulation has been considered in order to realize the greatest number of channels (e.g., 100 channels or more), non-relay transmission of 20 km or more, a multi-distribution system, and a high image quality transmission. In a DFB laser realizing such an advanced image transmission, a low distortion characteristic at a higher optical output and a wider modulation frequency bandwidth, compared with the conventional example, are required. As described above, in order to realize a low distortion characteristic at a high frequency bandwidth, the relaxation oscillation frequency of the DFB laser needs to be increased. In recent years, technology for increasing the relaxation oscillation frequency has been extensively studied. An example of this technology is the introduction of strain into well layers in a multi quantum well structure (MQW) (J. Lightwave Technol. LT-4 (1986) 504 and others).
A strained MQW-DFB laser, in which 0.7% compressive strain is induced into only well layers of the semiconductor laser having the conventional structure, is fabricated. A relaxation oscillation frequency characteristic is compared between the strained MQW-DFB laser and the unstrained MQW-DFB laser. FIG. 8 shows the results. The horizontal axis of the graph of FIG. 8 represents the number of well layers, and the vertical axis represents the relaxation oscillation frequency normalized with an injection current amount above a threshold value (normalized relaxation oscillation frequency).
FIG. 8 shows that as the normalized relaxation oscillation frequency is higher, a higher relaxation oscillation frequency can be obtained at a lower injection current amount. As is understood from the comparison between the strained semiconductor laser and the unstrained semiconductor laser, no difference in normalized relaxation frequency is found regardless of the number of well layers. That is, the effects of the introduction of strain are not obtained.
As described above, in the conventional semiconductor laser, in order to realize a low threshold current characteristic, InGaAsP having a bandgap wavelength of .lambda. g of 1.1 .mu.m or more is used as a material for the optical wave guide layer and the barrier layers. However, when InGaAsP having such a large bandgap wavelength is used for the barrier layers, the difference in bandgap energy (.DELTA. Eg) between the well layer and the barrier layer in a 1.3 .mu.m wavelength MQW structure becomes very small.
In the case where InGaAsP having a bandgap wavelength .lambda. g of 1.15 .mu.m is used for the barrier layers, .DELTA. Eg becomes 124 meV; in particular, the difference in bandgap energy (.DELTA. Ec) of a conduction band becomes a small value (i.e., 50 meV). In this case, as shown in FIG. 4A, electrons 19 injected into the well layers 14 overflow the well layers 14 to barrier layers 15 because of a small .DELTA. Ec. Consequently, the electrons 19 overflowing the well layers 14 are present in the optical wave guide layers 12 and 13. The electrons 19 present in the optical wave guide layers 12 and 13 cause the fluctuation of a refractive index due to the modulation, resulting in an increase in the amount of chirping. Such a fluctuation of the refractive index causes the above-mentioned axis-direction hole burning effect, leading to the deterioration of distortion characteristics. In addition, the overflown electrons 19 deteriorate the effects of the strain introduction into the well layers 14. Thus, even though strain is introduced into the well layers 14, the relaxation oscillation frequency is not increased.
As described above, the conventional MQW-DFB laser in which the optical wave guide layer and the barrier layers are made of InGaAsP having a bandgap wavelength .lambda. g of 1.15 .mu.m has the following problems:
(1) A low distortion characteristic is not likely to be obtained. PA1 (2) A low chirping characteristic is not likely to be obtained. PA1 (3) The introduction of strain into the well layers does not increase the relaxation oscillation frequency.
These problems have prevented a DFB laser having a wide bandwidth, low distortion characteristic and no transmission deterioration from being fabricated with high yield.